1. Field of the Invention
The present invention relates to a pattern inspection apparatus. More specifically, the invention relates to a pattern inspection apparatus for performing an inspection of a mask pattern, such as a mask, reticule or the like to be used in exposure technology for transferring a fine circuit pattern onto a semiconductor substrate or of a fine circuit pattern formed on a wafer. More specifically, the invention relates to a pattern inspection apparatus for inspecting a pattern configuration or a defect or an error in a pattern configuration of an inspection sample, such as a mask to be used for X-ray exposure technology for transferring a fine circuit pattern or the like onto a semiconductor substrate employing an X-ray beam including a synchrotron radiation beam (SOR), a wafer including a fine circuit pattern formed on a semiconductor substrate or a mask, reticule or the like to be used in light exposure technology for transferring a fine circuit pattern to a semiconductor substrate employing an ultra-violet light source.
2. Description of the Related Art
Conventionally, in order to transfer a fine circuit pattern onto a semiconductor substrate, ultra-violet light is used as a light source. Normally, a mask containing patterns for several chips is employed to perform a transfer of a compressed pattern onto a large diameter wafer by periodically shifting the transfer position on the wafer in a step-and-repeat manner. At the present time, for inspection of wafer patterns to detect foreign matter or defects on the mask pattern or to defect foreign matter on the wafer or the wafer pattern, an optical method employing an optical micrograph is frequently used. However, as the density of fine circuit patterns has become greater, it has become necessary to provide a light source with a shorter wave length for effecting wafer pattern inspection. In a conventional light source employing a conventional optical system, resolution cannot be improved because of specific diffraction limits which make it difficult to detect small defects.
To solve such a problem, electron beam inspection technology has been developed.
Thus, to facilitate the increasing density of circuit patterns, X-ray exposure technology employing an X-ray containing synchrotron radiation beam (SOR) is regarded as one of the important next age transfer technologies. Simultaneously, it is becoming necessary to shorten the wavelength of the light source used for inspection of X-ray masks.
In the prior art, the object of the inspection is to determine the presence of foreign matter adhering on the surface of the mask, reticule, wafer or the like, or to locate defects in a pattern such as the protrusion 201, intrusion 202, break 203, bridge contact 204, pin spot 205 or pin hole 206 or the like as shown in FIG. 18. The generally required sensitivity is, for example, in the case of foreign matter,
for a bare wafer: (pattern dimension).times.1/7 to 1/5
for a patterned wafer: (pattern dimension).times.1
for a reticule or mask: transfer limit.
On the other hand, in the case of a pattern defect, the required sensitivity level is generally (line width).times.1/2.
With increased pattern and package density, higher sensitivity becomes necessary. However, in the case of an inspection process employing a light beam, the extent to which the size of a light beam may be reduced is limited. Even when the wavelength is shortened, the highest possible sensitivity of visual light is 0.25 .mu.m. Therefore, the size of the device to be inspected is limited to 4M to 16M DRAM. On the other hand, even when an ultra-violet light beam is employed, the maximum possible sensitivity to be achieved is 0.15 .mu.m. In this case, the size of the device to be inspected can be up to 64M DRAM. Therefore, it is difficult to effect an inspection of a 256M DRAM or a next age device having further reduced size and greater packing density, such as a 1G DRAM.
Therefore, a method employing an electron beam in place of the light beam has been considered.
In methods employing electron beams, fewer problems will result from increasing the pattern density and thus higher density patterns can be achieved than with a light beam.
However, in prior art electron beam processes, there is a problem in that the information detected by a detecting means is processed independently in a time sequence in a detecting process and therefore, a relatively long data processing period is required.
In addition, as set forth above, when circuit pattern and package density are increased, the amount of data to be processed during pattern inspection of the device is naturally increased. Therefore, it is required to provide a greater capacity for accepting data to be arithmetically processed.
For example, there is a tendency to require a process for (1 cm/0.1 .mu.m).sup.2 =100 M pixel/cm.sup.2, 8" wafer, 31G pixel/wafer.
In contrast, when the density of the circuit pattern is increased, the size of the pattern unit for processing to check whether a defect is present or not becomes small. In addition, the capacity of the individual chip is increased. Therefore, where there is an increase in circuit pattern density, the amount of information to be processed for inspection rapidly increases. In conventional electron beam systems, a single electron beam is scanned across the mask in order to detect generated secondary, backscattered or transmitted electrons. Therefore, the signal stream from the irradiating region of an electron beam having high convergence for detecting very small defects, is generated continuously in a time sequence. A long time period is thus required for the transmission of the image information to a signal processing system. Therefore, even when the speed of the signal processing system is increased, a long time period is still required for inspection.
In a pattern inspection process employing an electron beam, the electron beam is irradiated onto an inspection sample, such as a mask or wafer or the like and the beam is scanned across the sample in order to generate an electron flow at the irradiated portion and introduce such electron flow into an electron detecting portion. The electron flow introduced into the detecting portion contains configuration information relating to the pattern. The signal contained in the electron flow provides time sequential pattern information.
The electron flow to be introduced into the detecting portion from the irradiated portion of the inspection sample may include secondary electrons which vary in quantity as function of the direction of incidence to the detecting portion depending upon the configuration of the inspection sample, backscattered electrons which vary in amount depending upon the material and configuration of the inspection sample or transmitted electrons which vary in amount depending upon the pattern of the inspection sample when the latter is a thin film mask or an X-ray mask or the like. From these respective electrons, the plane configuration and three dimensional configuration, such as projections and recesses and the like of the sample, can be recognized.
However, as set forth above, methods employing electron beams present problems in that the inspection speed is slow requiring a long processing period.